(1) Field of the Invention
The present invention is a method for the measuring the complex sensitivity of a hydrophone data channel in a line array at a frequency less than a minimum operating frequency of acoustic projectors.
(2) Description of the Prior Art
Calibration of a hydrophone, or array thereof, is often performed using a free field comparison method (See “American National Standard: Procedures for Calibration of Underwater Electroacoustic Transducers” ANSI/ASA S1.20-2012). In the method, one or more calibrated reference standard hydrophones with known free field sensitivity are used to measure the acoustic field that is transmitted by an acoustic projector at one or more locations in the vicinity of the hydrophone to be calibrated (i.e., the test article). The free field voltage sensitivity of a hydrophone channel is defined for the purpose of this invention as the ratio of the complex voltage output by the hydrophone to the complex acoustic pressure acting on the hydrophone. Thus, the free field voltage sensitivity is a complex quantity with both a magnitude and a phase. The magnitude is traditionally expressed as decibels relative to one volt per micropascal (e.g. dB re 1V/uPA). The phase is expressed in degrees relative to the incident acoustic field pressure.
Methods taught by the prior art for the comparison calibration of a hydrophone require the use of an acoustic projector to generate signals for comparison between the output of the test article and the output of the calibrated reference hydrophone. The projectors are capable of transmitting low frequency sound underwater; however, existing projectors do not generate acoustic signals that are suitable for a measurement of complex sensitivity at a very low frequency, or less than approximately 20 Hz.
A solution to the problem of an inadequate source level for an acoustic projector at a very low frequency is to perform calibration measurements in a comparatively small, confined, and controlled acoustic domain. Podalak (U.S. Pat. No. 2,918,651) teaches a method for hydrophone calibration at a very low frequency. In operation, the cited reference teaches a method where an acoustic projector is used to generate an acoustic field inside a rigid walled tube containing the test article (hydrophone) and the calibrated reference hydrophone.
Various refinements have been incorporated to improve accuracy and to allow for calibration measurements throughout a range of temperatures and pressures (See L. G. Beatty, R. J. Bobber and D. L. Phillips “Sonar Transducer Calibration in a High-Pressure Tube”, J. Acoust. Soc. Am., 39, pp 48-54 (1965) and J. F. Zalesak and W. J. Trott, “Low-Frequency Technique for the Underwater Calibration of Individual Elements of a Line Hydrophone Array”, J. Acoust. Soc. 62, pp 91-101 (1977)). While these methods may be effective; the methods are performed in a laboratory setting and are not generally applicable for hydrophones installed in end use equipment due to the requirement of the method to place the equipment in a comparatively small and confined space.
A different approach to the projector source level problem is disclosed in van Hoesen (U.S. Pat. No. 3,257,839) in which groups of three, closely-spaced acoustic transducers are integrated into an end use system. Known reciprocity calibration methods are employed for each of the hydrophones (See R. J. Bobber, “Underwater Electroacoustic Measurements”, Peninsula Publishing, Los Altos (1988)). Because each hydrophone in the array includes an integral primary calibration system; the free field voltage sensitivity can be measured over the life of the system without removing the array from service. However, the system is not useful for the calibration of hydrophones outside the integrated hydrophone data channels of the system.
The McEachern reference (U.S. Pat. No. 4,441,173) teaches a method to perform hydrophone calibration measurements at a very low frequency. In the reference, the calibration signal is generated by harmonically varying the hydrostatic pressure to which the test article and calibrated reference hydrophone are exposed. The time varying, hydrostatic pressure is generated by installing the acoustic receivers (a test article and a reference hydrophone) on a lever submerged in a body of water. A mechanism is used to harmonically vary the elevation of the acoustic receivers relative to the free surface of the water at a very low frequency. The sensitivity of the test article is provided by comparison with a calibrated reference standard hydrophone. While this measurement method eliminates the problem of an inadequate source level of standard acoustic projectors; it is not a feasible approach for the calibration of hydrophones that are integrated in larger systems.